Packet Size: Network MTU Vs. Maximum TCP

Low TCP packet size affects performance adversely

A young network administrator looks at a server rack.

Hero Images / Getty Images

The maximum transmission unit is the maximum size of a single data unit of digital communications that can be transmitted over a network. The MTU size is an inherent property of a physical network interface and is usually measured in bytes. The MTU for Ethernet, for example, is 1500 bytes. Some types of networks, such as token rings, have larger MTUs, and some networks have smaller MTUs, but the value is fixed for each physical technology.

MTU Versus Maximum TCP Packet Size

Higher-level network protocols like TCP/IP can be configured with a maximum packet size, which is a parameter independent of the physical layer MTU over which TCP/IP runs.

Many network devices incorrectly use the terms interchangeably. On both home broadband routers and Xbox Live-enabled game consoles, for example, the parameter called MTU is, in fact, the maximum TCP packet size and not the physical MTU.

In Microsoft Windows, the maximum packet size for protocols such as TCP can be set in the Registry. If this value is set too low, streams of network traffic are broken up into a relatively large number of small packets, which adversely affects performance. Xbox Live, for example, requires the value of the packet size to be at least 1365 bytes. If the maximum TCP packet size is set too high, it exceeds the network's physical MTU and degrades performance by requiring that each packet be subdivided into smaller ones — a process is known as fragmentation. Microsoft Windows computers default to a maximum packet size of 1500 bytes for broadband connections and 576 bytes for dial-up connections.

MTU-Related Problems

Transmission Control Protocol (TCP) Header

In theory, the limitation of TCP packet size is 64K (65,525 bytes). This limit is much larger than you'll ever use because the transmission layers have much lower sizes plus protocol overhead; TCP, for example, reserves 20 to 60 bytes just for the header of each packet. Ethernet's MTU of 1500 bytes limits the size of packets that traverse it. Sending a packet that is larger than the maximum transmission window for Ethernet is called jabbering. Jabber can be identified and prevented. If unaddressed, jabbering can disrupt a network. Usually, jabber is detected by repeater hubs or network switches that are designed to do so. The simplest way to prevent jabber is to set the maximum size of a TCP packet to no more than 1500 bytes.

Performance problems may also arise if the TCP maximum transmission setting on the home broadband router differs from the setting on individual devices connected to it.